Recently, with a large capacity of an optical disk, a demand for the optical disk is increasing in the fields of an auxiliary storage device of a computer, a consumer video recorder, and the like. With increasing the storage capacity, there is a demand for high-speed recording and high-speed reproduction.
When a recording mark is formed on the optical disk, emission intensity (hereinafter, referred to as a laser power or a power) of a semiconductor laser is set to a plurality of levels and modulated by a recording signal. The plurality of levels depend on a kind of a recording medium of the optical disk. FIG. 16 illustrates a relationship between the laser power and the recording mark with respect to a write-once optical disk by way of example. FIG. 17 illustrates a relationship between the laser power and the recording mark with respect to a rewritable optical disk.
During the recording, the example of the write-once optical disk of FIG. 16 has two kinds of levels of the emission intensity, namely, a bottom power and a peak power. The recording mark is recorded on a recording track in a multi-pulse emission interval in which an optical output is modulated between the peak power and the bottom power. The multi-pulse emission is adopted in order to more correctly form the recording mark. However, the multi-pulse emission can be eliminated depending on the recording medium or a recording condition of the optical disk. In the case of the write-once optical disk, the signal is neither written in nor deleted from the recording medium during the interval in which the emission is not performed at the peak power.
During the recording, the example of the rewritable optical disk of FIG. 17 has three kinds of levels of the emission intensity, namely, the bottom power, a bias power, and the peak power. The recording mark is recorded on the recording track in the multi-pulse emission interval in which the optical output is modulated between the peak power and the bottom power. The multi-pulse emission is adopted in order to more correctly form the recording mark. A recording space is recorded on the recording track in the interval of the constant bias power.
An example of a laser power control apparatus used in the rewritable optical disk of FIG. 18 will be described below.
FIG. 18 is a block diagram of a conventional laser power control apparatus. Semiconductor laser 200 is driven by laser driver 210. Laser driver 210 includes peak current source 201, bias current source 202, bottom current source 203, and modulators 204, 205, and 206 corresponding to the current sources. Peak current source 201, bias current source 202, bottom current source 203 are controlled by a control signal. Calculator 220 calculates the control signal based on a signal referring to the optical output of semiconductor laser 200. Peak current source 201, bias current source 202, bottom current source 203 supply a peak driving current Ipk, a bias driving current Ibs, and a bottom driving current Ibm to set the peak power, the bias power, and the bottom power, respectively.
Modulators 204, 205, and 206 are controlled to turn on and off the peak driving current Ipk, the bias driving current Ibs, and the bottom driving current Ibm according to the recording pulse generated by recording pulse generator 230. The turned-on and turned-off driving currents are supplied to semiconductor laser 200 after added.
FIG. 19 illustrates a relationship between the driving current of the conventional laser power control apparatus and an optical waveform.
FIG. 19(a) illustrates the optical waveform of the laser during the recording. During the recording, as illustrated in FIGS. 19(b), 19(c), and 19(d), a bottom modulated signal, a bias modulated signal, and a peak modulated signal are modulated to switch the driving currents to the laser according to a recording signal.
As illustrated in FIG. 19(e), the bottom driving current Ibm supplied by bottom current source 203, the bias driving current Ibs supplied by bias current source 202, and the peak driving current Ipk supplied by peak current source 201 are modulated by the bottom modulated signal, the bias modulated signal, and the peak modulated signal and added to become a final laser driving current.
In the above configuration, the recording mark is formed in the multi-pulse emission interval in which the peak modulated signal and the bias modulated signal are switched, and the recording space is formed when the laser power is kept constant while the peak modulated signal and the bias modulated signal are not switched. Specifically, in the multi-pulse emission interval in which the recording mark is formed, the driving current in which the peak driving current Ipk and the bias driving current Ibs are added with the bottom driving current Ibm being used as the base driving current is switched by the peak modulated signal and the bias modulated signal (for example, refer to Patent Literature 1).
A frequency of the recording signal is increased with the large capacity of the optical disk and the high-speed recording. Due to the multi-pulse emission, the frequency of the optical pulse is further increased, and a pulse width becomes as short as several nanoseconds or less. In the conventional laser power control apparatus, when the signal having the short pulse width is driven, the waveform of the optical pulse is deformed from a rectangular shape to a trapezoidal shape or a triangular shape by influences of a rise time Tr and a fall time Tf of the optical pulse. The rise time Tr and the fall time Tf of the optical pulse influence temperature control when the recording medium of the optical disk is rapidly melted and cooled. When the rise time Tr and the fall time Tf are lengthened, as a result, unfortunately the proper temperature control is hardly performed at the recording medium of the optical disk, and the high-speed recording is hardly performed.